1. Field of the Invention
The present invention relates to duplexers used for, for example, mobile communication terminals. More particularly, the present invention relates to a duplexer including a transmission filter with a ladder circuit configuration that includes multiple elastic wave resonators.
2. Description of the Related Art
Duplexers using surface acoustic wave filters have hitherto been widely used in, for example, mobile communication terminals. For example, Japanese Unexamined Patent Application Publication No. 2004-96250 discloses a surface acoustic wave filter having a ladder circuit configuration as a transmission filter of a duplexer. In Japanese Unexamined Patent Application Publication No. 2004-96250, an antenna terminal, a transmission terminal, and a ground terminal are provided on a piezoelectric substrate. The transmission filter having the ladder circuit configuration is provided between the antenna terminal, the transmission terminal, and the ground terminal. In the ladder circuit configuration, a wiring line connected to the ground terminal is arranged near the antenna terminal between the ground terminal and the antenna terminal to form a coupling capacitance Cg. The attenuation near a high-frequency side of a pass band is supposed to be increased due to the presence of the coupling capacitance Cg.
The provision of the coupling capacitance in the manner described in Japanese Unexamined Patent Application Publication No. 2004-96250 allows the attenuation near the high-frequency side of the pass band of the transmission filter to be increased.
Filter apparatuses in which longitudinally coupled resonator-type elastic wave filters are connected to the antenna terminals via one-port elastic wave resonators have been widely used as reception filters of the duplexers. The provision of the coupling capacitance described in Japanese Unexamined Patent Application Publication No. 2004-96250 in the transmission filter causes a problem in that isolation characteristics in the pass band of the transmission filter are deteriorated. The provision of the coupling capacitance described in Japanese Unexamined Patent Application Publication No. 2004-96250 in the transmission filter also causes a problem in that the attenuation outside the pass band of the reception filter is not sufficiently increased.